A partition between a user’s computer and the network/internet to block unauthorized access is known as:

The term you're looking for is a "firewall." A firewall acts as a security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Its primary purpose is to establish a barrier between a trusted internal network (such as a user's computer) and untrusted external networks (like the Internet). Firewalls can be hardware-based, software-based, or a combination of both, and they are essential for protecting sensitive data from cyber threats and unauthorized access. In modern computing, firewalls can also have advanced features such as intrusion detection, VPN support, and logging of network activity.